1. Field of the Invention
The present invention relates to a control apparatus for an internal combustion engine that serves to detect the presence or absence of knocking of the internal combustion engine (e.g., automotive gasoline engine, etc.).
2. Description of the Related Art
A typical known knocking detection apparatus for an internal combustion engine includes a knock sensor that detects vibrations generated in the internal combustion engine, a filter circuit that is connected to the knock sensor, and has a plurality of filters for extracting signals of different frequency bands, respectively, and a filter control circuit that selects one of outputs of the individual filters in accordance with the operating state of the internal combustion engine and outputs the thus selected one from the filter circuit (see, for example, a first patent document: Japanese patent application laid-open S56-637).
This knocking detection apparatus detects the presence or absence of knocking of the internal combustion engine based on the signal level of the output signal from the filter circuit.
In general, when the operating state of the internal combustion engine (e.g., the internal combustion engine is in a state of high speed operation or in a state of low speed operation or the like) changes, the signal level of noise and knocking contained in a signal of a frequency band extracted by a filter changes. That is, when the engine operating state changes, the frequency band suitable for detection of knocking changes.
Accordingly, provision is made for a filter circuit that has a plurality of filters for extracting signals of mutually different frequency bands, respectively, so as to make the detectability of knocking in the individual operating states optimal, whereby the presence or absence of knocking of the internal combustion engine can be detected with a high degree of precision by selecting one of the outputs from the individual filters in accordance with the engine operating state.
However, in case where the filter circuit is provided with the plurality of filters, there is the following problem. That is, it is necessary to provide filter component elements corresponding in number to the filters and to set a multitude of resistance values and the capacitors' capacitance values in accordance with the characteristics of the filters.
Accordingly, in order to solve the above-mentioned problem, a known knocking detection apparatus for an internal combustion engine includes a knock sensor that detects vibrations generated in the internal combustion engine, a switched-capacitor filter that extracts a signal of a frequency band peculiar to knocking from an output signal of the knock sensor, and a filter control circuit that switches an extraction frequency band of the switched-capacitor filter by adjusting a switching drive frequency supplied to the switched-capacitor filter in accordance with the operating state of the internal combustion engine (see, for instance, a second patent document: Japanese patent application laid-open H5-306645).
However, when one output from among the individual filters is selected in accordance with the engine operating state, or when the extraction frequency band of the switched-capacitor filter is switched over in accordance with the engine operating state, there is generated a large variation in the output signals from the filters or in the output signal from the switched-capacitor filter resulting from the switching of the frequency band.
Thus, for example, in case where the output signals from the filters are subjected to averaging processing for predetermined ignition cycles and then multiplied by a predetermined value, and the value thus obtained is set as a determination level for detection of the presence or absence of knocking, a considerable time is required until the time this determination level reaches a level with which the presence or absence of knocking can be normally detected after the extraction frequency band is switched over.
Accordingly, there has been a problem that the presence or absence of knocking is unable to be detected normally or correctly for such a period of time.
In order to solve the problems as referred to above, there has been proposed a known knocking detection apparatus for an internal combustion engine which includes a background level calculation section that calculates a background level corresponding to a level change of an output signal form a bandpass filter by averaging the output signal, a determination level setting section that sets a determination level for detection of the presence or absence of knocking based on the background level, and an update amount increasing section that forcedly increases an update amount for updating the background level or forcedly decreases an anneal number (reflection coefficient) set in the background level calculation section immediately after switching of an extraction frequency band of the bandpass filter (see, for example, a third patent document: Japanese patent No. 3341391).
In general, the output level of the output signal from the filter varies according to individual extraction frequency bands, respectively, so when the extraction frequency band is switched over, some time will be required until the background level becomes stabilized. Accordingly, the detection accuracy of knocking reduces during such a period of time.
Thus, by forcedly increasing the amount of update of the background level, or by forcedly decreasing the number of anneals (average changes of signal level) immediately after the switching of the extraction frequency band, the time required until the background level is stabilized is shortened.
Here, in case of increasing the amount of update of the background level, or decreasing the number of anneals (average changes of signal level), if there is a large difference in the signal level between the output signals before and after the extraction frequency band is switched over, a certain time is required until the background level is stabilized, so there exists a period of time in which the detection accuracy of knocking is low.
In addition, in case of increasing the amount of update of the background level, when the amount of update is set large so as to adjust the background level to an appropriate one immediately, there might occur an excess or deficiency of the amount of update due to variation in the output level of the individual knock sensor thereby to reduce the detection accuracy of knocking since the amount of update is set in absolute voltage terms.
For example, for a knock sensor having an output level of about half of a center characteristic thereof, it is necessary to set the amount of update thereof to ½, whereas for a knock sensor having an output level of about twice of the center characteristic, it is necessary to set the amount of update thereof to 2-fold. Accordingly, it is difficult to set an appropriate amount of update for the variation of the output level of the knock sensor.
In addition, the change of the background level becomes sensitive with respect to a change in the output of the knock sensor, the detection accuracy of knocking might be reduced due to transient response.
Thus, in the known knocking detection apparatuses for an internal combustion engine, there is the following problem. That is, when the extraction frequency band is switched over again after switching of the extraction frequency band during the time until the background level is stabilized, the background level is unstable, and the reduced detection accuracy of knocking continues.